CacheControl does not deal with Vary / secondary keys

[wsargent]

CacheControl does not have the concept of secondary keys as described in overview of cache operation

If a request target is subject to content negotiation, its cache entry might consist of multiple stored responses, each differentiated by a secondary key for the values of the original request's selecting header fields (Section 4.1).

Secondary keys are calculated from the Vary header as described in Calculating Secondary Keys with Vary.

This is mostly an issue when dealing with resources which use explicit content negotiation, which has failed miserably, but... it is part of the spec.

More details in https://www.smashingmagazine.com/2017/11/understanding-vary-header/

Also see how current browsers handle the fine points of Vary: https://wpt.fyi/results/fetch/http-cache/vary.html?label=stable&aligned

Other hints: https://www.mnot.net/blog/2017/03/16/browser-caching#vary

Discussion of "secondary keys" (a term completely undefined in the RFC itself) tempesta-tech/tempesta#508 -- part of the issue I have with the spec is that it assumes that you are working with a cache which even provides primary and secondary keys. As The Insanity of the Vary Header says:

This isn't really a major problem for developers writing clients and servers, but it's a pain for people trying to write caches. In fact, I haven't found a private cache implementation that actually does this yet.

and

If you think the specification is bad, you should see the implementations.

So even when CacheControl tells you to do the right thing, you're still going to be hosed unless your cache implementation can deal with the ambiguity: the cache entry can be either a single response, or a map of multiple stored responses.

val entry: Entry = cache(primaryCacheKey)
val response: Response = if (entry.containsMultiple) {
  entry.response(secondaryKey)
} else {
  entry.response
}

Do you understand the Vary HTTP header?

Also see httpwg/http-core#110 and httpwg/http-core#28

https://developer.mozilla.org/en-US/docs/Web/HTTP/Caching#Varying_responses

https://dzone.com/articles/getting-the-most-out-of-vary-with-fastly